The present invention relates in general to a dither noise source for reducing quantization error in digital systems, and more specifically, to generating a dither noise signal with a notched frequency spectrum.
The use of dither is a well known technique for reducing distortion in digitized signals that otherwise arises when low level analog signal variations smaller than the quantization interval are present in an analog signal being digitized by an analog-to-digital converter. The addition of a small analog noise signal to the analog signal being digitized randomizes the quantization error, thereby reducing the distortion. Although a small level of white noise is added to the signal, the effect on the audible quality of sound is much less objectionable than the distortion. It is known, furthermore, that the noise effects of dither can be reduced by spectrally shaping a dither noise signal to avoid frequency content in the main frequencies of interest in the signal being digitized. Such spectral shaping, however, has required excessive processing to generate an acceptable dither signal. In order to make generation of such spectrally-shaped dither noise practical, an efficient manner of generating the dither noise is needed.
In the case of an AM/FM radio receiver in which analog intermediate frequency signals are digitized for demodulation and subsequently processed in a digital signal processor (DSP), the analog signal of interest has a different frequency spectrum depending upon whether AM or FM is being received. AM reception typically utilizes an intermediate frequency of 450 kHz while FM reception typically uses an intermediate frequency of 10.7 MHz. Thus, a dither noise source for adding to the intermediate frequency signal would need to have a spectral shape avoiding these two separate frequency bands. Relying solely on filtering of a white noise dither signal would be extremely inefficient and costly.